How to turn off camera's follow path constraint?
Hm, this is another "follow path". Try to make "follow path" in different way: select Curve, Shift-select Camera, then press  Ctrl+Shift+C and select Follow Path. Then chose: in constraint settings click on "Animate path" or animate "Offset" value. This is equivalent of  "follow path" via constraint.



Also, you can add a constraint from menu Add Object Constraint, but in this way you need to manualy select curve object in "Target" drop-down selector.

To remove default "follow path" go in curve settings and unclick Path Animation checkbox.